In numerical transmission systems the data binary stream which will be sent to the distant terminal is initially converted into a three level code (AMI, HDB3, etc.) which has the function of introducing polarity variations, whenever bit sequences with the same logical value are present in the binary signal. The data stream, coded on three levels, is usually also inputted into another coding set designed to output the signal that is sent onto the transmission line. As a line code, a 1B/2B type code (eg: MCMI) is usually used, in other words of the type in which each of the symbols of the three level code is converted into a pair of bits.
Further on in the present description, purely as an example, reference will be made to a line terminal which uses the HDB3 code as a three level code, and the MCMI code as a line code. In the MCMI code the symbols are transmitted expressed in the HDB3 code according to the following rules:
the "+1" of the HDB3 code are transmitted as "1+1" PA0 the "-1" of the HDB3 code are transmitted as "0-0" PA0 the "0" of the HDB3 code are transmitted as "0-1" PA0 separation means, designed to receive at its input the signal coded according to the the line code and also designed to output on a first output, and respectively a second output, binary streams obtained by serializing the first bits, and respectively the second bits, of each word of the line code; PA0 first means for detecting bit errors, connected to the first output of the separation means, designed to activate their own output each time they receive at the respective input a pair of bits with a logical value of "one"; PA0 second means for detecting bit errors, connected to the second output of the separation means, designed to activate their own output each time they receive in input a pair of bits with a logical value of "nought"; and PA0 error totaling means designed to provide indications about the total number of pulses that correspond to the output of the first and second means for detecting errors;
It can be noticed that the "1-0" configuration is never used in the MCMI code (forbidden configuration), so that it can be used for measuring the errors made during transmission of the information stream.
In fact, there is usually a circuit present in the receiving section of the line terminals which is specifically for measuring the error rate and which provides indications regarding the quality of the transmission as described above.
There are also well known circuits for measuring the error rate which however have the inconvenience of being so designed as to only pick up a fraction of the errors actually made. These circuits operate using the "forbidden word" method If some errors are made in the receiving station during recognition of the logical levels, some of these errors give rise to the forbidden word "1-0". Therefore, the method in question is based on detecting the forbidden configuration and counting of the number of times it is detected in a predetermined interval of time. But it must be kept in mind that the forbidden word can only be generated if the noise present in the line causes a wrong recognition of the second bit of the word "1-1" (code expression of the "+" symbol HDB3) or of the first bit of the word "0-0" (code expression of the "-" symbol HDB3). The forbidden word can never be generated with reference to the configuration "0-1" (code expression of the "0" symbol HDB3), except in the case of two consecutive errors (a condition which can occur only with an extremely low probability rate). Consequently, the forbidden word method makes it possible to pick up only about 27.5% of the effective errors made at the most. However, the detection of errors is only applicable under theoretical conditions, and in practice the percentage of errors that may actually be picked up is much less than that specified above. In fact, 27.5% of errors may be picked up on the condition that the line noise is of a Gaussian type, on the condition that the probabilities for committing errors are the same on the "1" bits as on the "0" bits, and on the condition that the probabilities for committing errors do not depend on the type of sequence that is transmitted on the line in a given instant.
In actual practice, it must however be kept in mind that in fiber optic systems the noise has a different spectrum from Guassian noise and that the noise level is a function of the signal level. Consequently, the signal to noise ratio S/N is worse in the presence of "1" bits, so that the probability of committing errors in the discrimination operation is greater for "1" bits than for "0" bits.
Furthermore, if the transmission medium attenuates the high frequencies more in respect to the low frequencies, then the probability of committing errors during recognition of the "0-1" configuration is all the greater with respect to the "0-0" and "1-1" configurations the more marked is the difference in attenuation described above.
The attenuation difference mentioned above, in actual practice, causes a greater probability of committing errors during recognition of the "0" symbol in HDB3 where, as has already been mentioned, the forbidden word is never generated. Therefore, the number of bit errors which theoretically may be measured with the forbidden word method (about 27.5%), in actual fact is greatly reduced in practice.
All that has been said so far is true when the transmission medium is a fiber optic and the noise is also generated by a photodetector.